The invention described herein relates generally to high speed communications over multi-wire cables, and more particularly to increasing the data rate of OFDM communications over multi-wire cables.
Digital Subscriber Lines (DSLs) provide high-speed Internet connections with higher data rates than previous dial-up lines. DSL exploits the fact that a telephone cable from a home to the exchange will support signals at much higher frequencies than the 300 Hz-3.4 kHz telephone audio signal. To enable conventional telephone operations to function simultaneously with DSL operations, DSL uses filters to divide the spectrum into DC+Audio for the conventional telephone and a high frequency range of about 100 kHz to 1 MHz, or even higher for advanced forms of DSL.
While DSL provides high-speed communications via copper-wire cables, conventional DSL and other copper wire-based communications cannot currently deliver the total data rate of approximately 40-60 Mb/s required to allow independent HDTV program selection on several different TVs in a single household. Instead, copper wire-based communications can only offer a very limited TV service, called IPTV, which has a limited data rate that limits the quality of the received programming and does not permit several TV programs to be viewed simultaneously in a home. In direct contrast, the higher bandwidths available to cable and satellite communications enable cable and satellite companies to offer full TV services for multiple TVs in a single household, including multiple HDTV services, in addition to regular voice and Internet services. As a result, cable and satellite communications companies currently have a competitive advantage over telephone companies that offer services via their traditional copper wire infrastructure. On the other hand, the advantage of a copper wire infrastructure is that it provides one or more copper wire pairs unique to each subscriber, which allows the selection of the service or program to be delivered to be made at the exchange.
There are a number of hindrances to increasing the data rate of DSL communications over copper wires. For example, the line attenuation of copper wires per unit length increases at higher DSL frequencies. Thus, as the distance from the exchange increases, the signal-to-noise ratio (SNR) at the higher frequencies decreases. SNR considerations thus either limit the maximum useable frequency, and/or limit the largest useable modulation constellation, and thus the bits/sec/Hz. Further, crosstalk between signals on different twisted pairs in the same cable also degrades the SNR. In addition, long, medium and short wave radio broadcasting also use the same frequency range used for DSL operations. Existing unscreened lines may therefore suffer from external interference caused by strong radio stations, which may blot out part of the spectrum.
Crosstalk and external interference typically increase the noise levels on multi-pair copper wire cables beyond the thermal noise (kT) level by a considerable amount. For example, the noise level of a multi-pair cable may be −140 dBm/Hz, whereas kT=−173 dBm/Hz. This 33 dB higher-than-thermal noise level limits the cable distances to the exchange to a few kilometers, and therefore, limits the number of bits/sec/Hz of bandwidth that can be successfully modulated and demodulated using higher order QAM signal constellations such as 256 QAM, and even 65536 QAM.
Current copper wire infrastructures may utilize multiple techniques to reduce the crosstalk and/or external interference. The most common technique for reducing crosstalk between DSL signals on different wires is called “Dynamic Spectrum Management” (DSM). DSM reduces crosstalk by avoiding the use of the same subcarrier in adjacent wires at the same time. While DSM is effective when fewer than all of the wires in a cable are used, DSM is significantly less effective when all wires are used, e.g., when the highest data rates are desired.
Another crosstalk reduction technique requires all DSL signals to terminate at the same user's modem. However, this technique is only available when all DSL signals in the cable terminate at the same end station, e.g., house, and thus does not apply to cables from curbside distribution boxes to the exchange, which carry DSL signals for different users.
Cable manufacturers may also consider crosstalk issues when manufacturing multi-wire cables, as explained in reference 1 “Overview Over Transmission Media I.” For example, the cable manufacturer may vary the disposition of each pair relative to every other pair cyclically or randomly so that any residual imbalance in either capacitive or inductive coupling is distributed evenly from a given pair to every other pair. A consequence of such a cable weave is that the capacitance from every wire to every other wire is nominally equal, except for the other wire of the same pair with which it is twisted over the entire cable length. Likewise, the mutual inductances between any wire and any other wire are nominally equal, except for the other wire of the pair with which it is twisted over the entire cable length. The degree of equality of inter-wire capacitances and mutual inductances is however very cable-dependent.
Despite all of these options, copper wire infrastructures currently cannot deliver the data rates necessary to provide full TV services, especially HDTV services, to multiple TVs in a single household. Thus, there is a great motivation for copper wire companies to explore ways to deliver data rates that will support delivery of multiple HDTV programs to multiple households, thereby allowing them to compete effectively with cable and satellite companies.